A tomodensitometric apparatus usually includes a source of radiation which emits the radiation in a diverging, fan-shaped beam which defines a sector of the tomographic plane in which the object to be examined is placed. The source and the detectors of the radiation are attached to a movable support which is capable of rotation about an axis which is perpendicular to the tomographic plane and is located substantially in the center of the object to be examined. The varying signals emitted by each of the plurality of detectors are received by a suitably programmed computer during the cycle of rotation. The computer processes these signals and derives from them information regarding the degree of absorption of radiation and forms an image of the examined object on the basis of the data.
In order for the calculations and the processing to be exact, i.e., to correspond to the objects actually examined, it is indispensable that the totality of the object be irradiated during the entire cycle, i.e., that all of the points of the object to be examined lie at all times within the sector defined by the source and the detectors. The foregoing condition may be expressed by saying that the object to be measured must lie within a so-called field of measurement, i.e., a circle whose periphery is defined by the envelope of rays intercepted from the source by the detectors lying at the extreme ends of the array of detectors as traced out during the rotation of the assembly. If the object to be examined did not lie inside such a circle, then at some times during the cycle of examination, parts of the object would not be measured, leading to an erroneous result. It is thus of the utmost importance to place the object at the interior of the aforementioned circle, an action which might be termed "centering".
This act of centering could be performed by simulating the measuring field with mechanical openings or by the generation of a luminous pattern with which the object to be examined could be aligned. However, in such a case these centering means would also have to be correctly centered on the measuring field, which presupposes lengthy preliminary operations and considerable care. Furthermore, the use of such systems tends to be complicated and slow. The object to be examined could also be correctly centered by means of light sources and light detectors in the optical part of the spectrum but such mechanisms would also require additional associated electronic circuits, either for use by the operator of the machine or for use by the computer.
In principle, it would even be possible to perform a test cycle using penetrating radiation and to make necessary corrections if the object were not correctly centered. However, it is to be noted that this procedure is definitely excluded due to the detrimental effects of the additional radiation received by the patient or body to be examined in such a test cycle.